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International Refereed Journal of Engineering and Science (IRJES) is a leading international journal for publication of new ideas, the state of the art research results and fundamental advances in all aspects of Engineering and Science. IRJES is a open access, peer reviewed international journal with a primary objective to provide the academic community and industry for the submission of half of original research and applications

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International Refereed Journal of Engineering and Science (IRJES)

  1. 1. International Refereed Journal of Engineering and Science (IRJES)ISSN (Online) 2319-183X, (Print) 2319-1821Volume 2, Issue 5(May 2013), 46 | PageStart-Up Of An Upflow Anaerobic Sludge Blanket ReactorTreating Low-Strength Wastewater Inoculated WithNon-Granular SludgeK.R.Venkatesh, M.Rajendran & A.MurugappanDepartment of Civil Engineering, Faculty of Engineering and Technology, Annamalai University,Annamalai Nagar- 608 002, Tamilnadu, India.Abstract :The upflow anaerobic sludge blanket (UASB) process is known to be energy conservativebiotechnology. Its low cost and low skill requirement render it to be a viable technology for reducing organicpollution loads. However, this system is facing a challenge in the treatment of low-strength wastewaterespecially inoculated with non-granular sludge. This work was performed to emphasis an in-depthunderstanding of an UASB reactor start-up process inoculated with digested slurry for treating syntheticwastewater of chemical oxygen demand (COD) around 700-1000 mg/L. The lab scale reactor with an effectivevolume of 9.97 L was operated under ambient temperature (24-350C). At the end of start-up period that lastedfor 84 days, this system achieved 90.8% COD removal and a biogas evolution of 4.72 L/d (457 L / kg CODremoval) at an organic loading rate (OLR) of 1.293 kg COD/ m3d. Concurrently, the volatile fatty acids (VFA)/alkalinity ratio was found as 0.184. It confirms the stability of the reactor.Keywords: alkalinity, anaerobic treatment, biogas, start-up, volatile fatty acids.I. INTRODUCTIONWastewater may be categorized as low-strength, medium strength, high strength and very high strengthbased upon the concentration of chemical oxygen demand (COD), bio-chemical oxygen demand (BOD),suspended solids (SS) and nutrients such as nitrogen (N) and phosphorus (P). Generally the stronger theeffluent, the greater the investment in infrastructure and energy inputs required to achieve environmentallysound treatment objective [1]. Several techniques are already available to attain the goals of environmentalprotection and resource conservation. In the case of wastewater treatment, combination of different methods can beused, viz. physical, chemical and biological. The last method, which constitutes the main units of wastewatertreatment plants, can be divided into two classes such as aerobic and anaerobic [2].Aerobic process, which are widely used for the treatment of wastewater have two distinct disadvantageslike high energy requirement and excess sludge production, which require handling, treatment and disposal [3]. Incontrast, anaerobic processes generate energy in the form of biogas, and produce sludge in significantly loweramounts than those resulting from aerobic systems. In the absence of molecular oxygen anaerobic processconvert organic material into methane, a fuel that can yield a net energy gain process operation. Anaerobicprocesses have gained popularity over the past 20 years. It have been already applied successfully for thetreatment of different type of wastewater and geared mainly towards high concentrated soluble wastewater [4].The most common reactor type used for anaerobic digestion of wastewater is the continuously stirred tankreactors (CSTR). The main problem of this reactor is the removal of active biomass from the system that leadsto long retention time. It has been overcome by immobilizing the active biomass, referred as high rate systems.A typical such reactor is upflow anaerobic sludge blanket (UASB) reactor [5].In the UASB process, wastewater flows through bed (granular or flocculent), where different physicaland biochemical mechanisms act in order to retain and biodegrade organic substances. Solubilisation andhydrolysis of SS is a slow process accomplished by extracellular enzymes excreted by acidogenic bacteria.Readily biodegradable substances are quickly acidified and then converted into methane and other biogascomponents [6]. The great success of the UASB process lies in its capability to retain a high concentration ofactive sludge due to granule formation. It allows the application of high loading rates and maintenance of highsolids retention time at a low hydraulic retention time (HRT) so that efficient treatment can be carried out in atemperature climate as well [7].Low-strength wastewater such as domestic sewage (COD concentration 500-1000 mg/L), are atpresent treated mainly aerobically. However, due to considerable operation and maintenance cost, studies arebeing conducted to develop economically more attractive high rate anaerobic systems [8]. UASB reactor systemis facing a challenge in the treatment of low-strength wastewater. Nevertheless, more efficient anaerobic
  2. 2. Start-up of an upflow anaerobic sludge blanket reactor treating low-strength wastewater 47 | Pagesystems have been developed and they are being successfully applied for treatment of low-strength wastewatersuch as domestic wastewater, particularly under tropical conditions where artificial heating can be avoided [9].The production and quality of granular/flocculent sludge is influenced by the composition of wastewater, reactordesign and technological condition. Moreover, the formation of granular/flocculent sludge with good settlingcharacteristics and activity is also a critical factor in dealing with low-strength wastewater [10]. In general, theoperational conditions induce an ecological selection that leads to the formation of microbial aggregates, the so-called ‘granules’ [11]. Granules are the best inoculum for the start-up of UASB reactors due to their highspecific activity and settleability. However, they are not always available. Consequently, very often the start-upprocess and the tentative granule formation have to be performed with an inoculum of diffuse sludge collectedin conventional anaerobic digesters. There is lack of data on the operating conditions during the start-up of aUASB reactor that uses diffuse sludge as the inoculum. On the contrary, when treating wastewaters with CODvalues lower than 1500 mg/L, which are low- strength wastewater in the anaerobic treatment field, the start-upof UASB reactors is difficult and thus much less full-scale reactors are in operation under these conditions thanin the case of medium and high strength effluents [9].Keeping this in mind, the experimental work here described was focused on the anaerobic treatment oflow-strength wastewater under tropical conditions. The main objective of this work was to look into the start-upprocess performance of a lab scale UASB reactor inoculated with digested slurry, which was collected from anactive biogas plant. The operational parameters such as COD, upflow velocity, total alkalinity, volatile fattyacids (VFA) concentration, total suspended solids (TSS), volatile suspended solids (VSS) and pH wereinvestigated. In addition to that, biogas generation and COD removal efficiency were also calculated.II. METHODS2.1 Experimental set-upUASB reactor employed in this study was fabricated using transparent plexiglass tube of 0.1 mdiameter vertical cylindrical shape with a total volume of 11.78 L. The reactor essentially had an internaleffective working volume of 9.97 L and the remaining volume of 1.81 L was kept for gas liquid solid (GLS)separation arrangement. The overall height of the reactor was 1.5 m and effective height being kept as 1.27m.The reactor had one influent port at the bottom, one effluent port and five sampling ports (Figure. 1). The GLSseparator attached at the top of the reactor was essentially a set of two inverted cones with intermediate overlapto facilitate effective biogas collection. Gas was collected from the outer inverted cone as well as below the deadcorners of the inner cone. The treated effluent collected by a pipe attached above the GLS separator wasconnected to a water seal to prevent the escape of gas. A peristaltic pump (Miclins PP 15) was used for feedingwastewater as uniformly as possible over the reactor bottom, passes through the sludge bed in the reactor. Thereactor was operated at mesophilic temperature (24 - 35°C).2.2 Synthetic wastewater preparationIn this study, synthetic wastewater was prepared, in which organic COD was provided by sucrose anddried milk powder. About 1000 mg/L of COD contains 750 mg of sucrose and 250 mg of dried milk powder.The ratio of soluble to particulate COD in the feed was around 80:20. The synthetic wastewater was thenneutralized to a value of pH ~ 7.2 by using sodium hydroxide (NaOH). Urea and KH2PO4were added as sourcesof nitrogen and phosphorus respectively to maintain COD: N: P ratio. A ratio of 300:5:1 was maintained asCOD: N: P during start-up period [12]. The pH in the feed tank was adjusted to a value ~ 7 after every 10 -12hours. The fresh feed was prepared daily. Trace elements like ferric chloride, copper sulfate, magnesiumchloride, calcium chloride were added appropriately.2.3 Seed sludgeDigested slurry was collected from an active biogas plant located at Faculty of Agriculture, AnnamalaiUniversity, Annamalai Nagar. After analyzing the concentration of total suspended solids (TSS) and volatilesuspended solids (VSS), this sludge was used as seed. Initially about 50% reactor volume was filled up with thisactive sludge.
  3. 3. Start-up of an upflow anaerobic sludge blanket reactor treating low-strength wastewater 48 | Page1. Feed inlet, 2. Peristaltic pump, 3. Effluent outlet,4. Biogas collection, 5. Gas measurement unit,6. Sampling ports, 7. Effluent collecting tank,8. 10 cm diameter and 150 cm tall plexiglass tube,9. Gas liquid solid separation systemFigure1. Schematic diagram of UASB reactor showing experimental setup2.4 AnalysisDuring the operation of the reactor, temperature, sludge bed height and biogas production weremonitored and recorded daily. Feed and treated effluent was taken for the analysis of COD, pH, total alkalinityand VFA on daily basis. TSS and VSS were determined once in a week. Liquid samples were collected from theinfluent, effluent and sampling ports along the reactor. Quality control was ensured using standards as well asduplicates while performing analysis. All the parameters were determined according to standard method for theexamination of water and wastewater [13]. COD was determined by open reflux method. VFA was determinedby distillation method and measured as acetic acid per liter. Biogas evolved was collected by water displacementmethod. The water in the gas collector was acidified with sulfuric acid to reduce the solubility of one of theconstituents of biogas carbon dioxide (CO2), for optimum measurement of biogas volume.III. RESULTS AND DISCUSSION3.1 Characteristic of seed sludge and inoculationInitially the seed sludge (digested slurry) had a TSS content of about 70,100 mg/L and VSS content ofabout 28,000 mg/L. Similar type of sludge was used as seed sludge for treating sago wastewater in a HybridUASB reactor [14]. Lettinga and Hulshoff Pol [15] revealed that in order to avoid excessive sludge washout, theamount of seed sludge must be small enough to maintain the sludge bed within the reactor upon increasing thespace-loading rate, and large enough to prevent a needless delay of the reactor start up. They also suggested,when using thick digested slurry of relatively low specific methanogenic activity, 12 to 15 kg VSS/ m3sludgecan be used because expansion washout will be limited in that case. While using s sludge with higher specific
  4. 4. Start-up of an upflow anaerobic sludge blanket reactor treating low-strength wastewater 49 | Pageactivity, the amount of seed sludge should not exceed approximately 6 kg VSS/ m3, because more seed sludgewill be washed out due to the occurrence of sludge bed expansion. Hence, initially about 50% reactor volumewas filled up with this active sludge having VSS concentration of about 28,000 mg/L. Accordingly the VSSconcentration in the entire reactor at the start-up was about 14,000 mg/L (14 kg VSS/m3). Hickey et. al., [16]recommended Sludge with 40% reactor volume might be enough to accelerate start-up.3.2. Reactor Start-upThe start-up period is considered as the period taken for stable operation to be achieved. This is acrucial step for the stable operation of the UASB and other anaerobic reactors, at a designed organic loading rate(OLR). One of the main points stressed is the inoculation with high quality methanogenic sludge. Sludge ofpoor quality can also be used [17]. If self-inoculation can be effected, the start-up periods can go up to orbeyond 6 months and other complications could arise [4]. In addition, operating temperature is prominent duringstart-up. In this work, the UASB reactor after seeding was operated at a temperature between 240C and 350C.This comes under the mesophilic range (200C to 450C). Fluctuations within this range were not enough to causeany real impact on performance and in terms of reaction kinetics [18].To begin with, the sludge has a VSS / TSS ratio of about 0.40. Poor generation of biogas at the initialstage of start-up shows that sludge activity was initially low. At the same time, feed concentration of less than1000 mg/L of COD helped in controlling the excessive volatile fatty acids (VFA) formation and guarded againstthe excessive pH drop. An inhibition of methanogenesis generally results in an increase of VFA concentrationand a sudden drop in pH [19]. The upflow velocity is also an important operational parameter in anaerobicdigesters. It maintains the mixing and hydraulic retention time of the substrate and biomass [20]. The upflowvelocity maintained initially was low of about 0.106 m/h. No recirculation was employed to allow the lighterfraction of biomass to wash out from the reactor. The upflow velocity maintained during start-up phase ispresented in Figure. 2. Accordingly, the OLR maintained during start-up phase is presented in Figure. 3.COD removal rate during first four weeks were low in the range of 30-35% (based on total influentCOD and total effluent COD) (Figure. 3). The low removal efficiency during start-up phase can possibly beattributed to the presence of unadapted seed sludge [10].Sufficient upflow velocities were maintained in thereactor, in order to facilitate sludge blanket formation offering higher area for contact between sludge andwastewaters. During period of 30 to 40 days higher upflow velocity of 0.128 m/h was tried (Figure. 2). Thisupflow velocity was preferred to permit the wash out of voluminous (poorly settling) sludge. This amounted tohigher OLR of more than 2.1 kg COD/m3d (Figure. 3). However, these operating parameters made start-upoperation unstable as evidenced by increase in VFA concentration in the effluent. Hence, the upflow velocityhad to be controlled to a value of about 0.102 m/h after day 44 (Figure.2).Many researchers suggest that the hydraulic retention times (HRTs) should not be allowed to less than6 h and it should be less than 18 h during start-up period to treat any type of wastewater. At lower HRT, thepossibility of washout is more prominent. This makes it difficult to maintain the effective number of usefulmicroorganisms in the system [21]. For this reason, after day 3, the HRTs were varied between 10 h and 18 hduring start-up period.Figure 2. Feed, effluent COD concentrations and upflow velocity maintained during start-up period0200400600800100012000 10 20 30 40 50 60 70 80 90Time,daysCOD,mg/L0.0000.0500.1000.1500.2000.250Upflowvelocitym/hCOD mg/L Influent COD mg/L EffluentUpflow velocity m/h
  5. 5. Start-up of an upflow anaerobic sludge blanket reactor treating low-strength wastewater 50 | PageFigure 3. Organic loading rate maintained and % COD removal during start-up periodAs discussed earlier during start-up period COD: N: P ratio was maintained as 300:5:1. N and Psupplementation was necessary characteristics. COD: N: P should be at least 300:5:1 for efficient rapid start-up[12], [22]. Likewise, neutralization by alkali to a value of pH 7.0 - 7.2 was also necessary, particularly duringstart-up. In addition, optimal methanogenesis is known to take place at a neutral pH. The well-being of a reactorcan often be judged by the examination of pH value of influent and the treated effluent. Variations in pH of theinfluent and effluent are presented in Figure. 4. The value of pH slightly fell during the initial days of operation,indicates the prevalence of acid fermentation over methanogens. However, after the initial drop, pH of thetreated effluent was in the range of 7.2 - 8.3. It indicates healthy anaerobic environment, which is favorable formethanogenic organisms [23].Figure 4. Feed and effluent pH during reactor start-up period01020304050607080901000 10 20 30 40 50 60 70 80 90Time, days%CODremoval0.0000.5001.0001.5002.0002.5003.0003.5004.0004.5005.000OLR,KgCOD/m3d% COD RemovalOLR, kg COD/ m3 d
  6. 6. Start-up of an upflow anaerobic sludge blanket reactor treating low-strength wastewater 51 | Page0501001502002503003504000 10 20 30 40 50 60 70 80 90Time, daysVFA,Alkalinity,mg/L00. mg/L EffluentVFA mg/L EffluentVFA/alkalinity ratioTotal alkalinity, VFA concentration and pH are effective indicators of the stability of thereactor and of the correct balance between acidification and methanogenesis. The acidogenic element of theoverall digestion processes will generate acids, but under normal conditions this would be buffered by thealkalinity in the reactor (expressed as calcium carbonate). Alkalinity may also be caused due to evolution ofCO2, during decomposition of organic matter [24]. In an anaerobic digester, a bicarbonate alkalinity (as CaCO3)of about 1000 to 3000 mg/L was required for stable operation [25]. Disparity to this, Van Haandel and Lettinga[26] shows an average alkalinity with a range of 220-390 mg/L for the successful operation of a UASB reactorover 200 days. On this basis, sufficient alkalinity was available in the reactor. The alkalinity value of the treatedeffluent considerably increases as the reactor progressing towards steady state conditions (Fig. 5). Methogenisis,in particular is known to become unstable when the VFA / alkalinity ratio value is above 0.3 [27], [28]. Fromthe beginning of the experiment, the effluent VFA values were less than 225 mg/L. The bicarbonate ionconcentration or bicarbonate alkalinity is approximately equivalent to the total alkalinity for most wastes whenthe volatile acid concentration is very low [29]. On this aspect, total alkalinity measured during the entire periodof operation represents bicarbonate alkalinity. In this work, VFA/alkalinity ratio after 40 days was 0.5. As thereactor progresses towards stabilization, the ratio gradually decreases. At the end of start-up period, the VFA/alkalinity ratio was found as 0.184 (Fig. 5). Most researchers revealed that a value of greater than 0.40 for thisratio indicate reactor instability.The VFA concentration (measured as acetic acid) in the effluent during first 30 days was in the rangeof 225-170 mg/L from that it fell down to 108 mg/L on day 50. Higher levels of VFA in the wastewaters duringthe initial phases of operation indicate the prevalence of acid fermentation [26]. Subsequently, the VFA in theeffluent decreased and was in the range of 80 to 59 mg/L towards the end of start-up period. It indicates healthyanaerobic environment and satisfactory methanogenic activity. The overall performance of the reactor duringthe start-up was more than satisfactory. The selection of seed material plays a crucial role in minimizing timerequired for initial bioflim establishment [30], [31]. It was likely that the slurry collected from an active biogasplant and used as a seed had sufficient numbers of physiologically active microorganisms.The biogas production up to 20 days was low, less than 1.4 L/d. Beyond day 48, however as soon asVFA concentration was controlled to less than 110 mg/L, the gas production as well as COD conversionefficiencies started showing marked improvement. By day 84, the gas production rate reached a value of about4.92 L/d (Figure. 6) and the VFA in the effluent had reduced to less than 60 mg/L (Figure. 5). This shows thatthe process of hydrolysis and acidification of the organic matter took place in proper condition.Figure 5. Effluent VFA, Alkalinity and VFA / Alkalinity ratio during start-up period.It was assumed that the reactor had reached steady state condition merely 78 days after inoculation.This is because the results of effluent COD presented the same pattern i.e. approximately the same average CODconcentrations and the same range of variations during the whole operation period after 78 days. There was no
  7. 7. Start-up of an upflow anaerobic sludge blanket reactor treating low-strength wastewater 52 | Pagerecycling of sludge during entire operation. Effluent recycle was not necessary to fluidize the sludge bed assufficient contact between the wastewater and sludge is guaranteed even at low organic loading rate in UASBreactor [32]. The start-up period for the reactor was completed in 84 days, as the VFA level in the effluentstabilized gradually. The pH value of the treated effluent and increase in total COD removal efficiency (90.80%)indicates good start-up.Determination of VSS/TSS ratio gives correlation to the biomass growth and its quality. The VSS/TSSratios based on the average TSS and VSS concentrations in the reactor along with substrate utilization wasobserved at different periods of operation. The biomass concentration profiles were obtained by the TSS andVSS concentrations taken from sampling ports at various heights of the reactor (data not shown). At the time ofreactor startup, VSS/TSS was 0.40. There was a steady increase in VSS/TSS ratio throughout the operation.The VSS/TSS ratio increased up to 0.64 by day 50 further increased to 0.80 on day 78. Initially the sludge bedheight inside the reactor was 0.70 cm. On day 50 and day 78, it was 0.40m and 0.45m respectively. However,the experiment with the sludge morphological study was conducted at the end of the operational period.Figure 6. Biogas production during start-up periodIV. CONCLUSIONSThe attributes gathered from this study points out the feasibility of UASB reactor for the treatment oflow-strength wastewater with COD values lower than 1000 mg/L, inoculated with non-granular sludge. Thedevelopment of sludge bed was observed during this start-up phase. Decrease in VFA values, increase in pHvalues and alkalinity of the effluent, increase in biogas production rate and COD removal efficiency suggeststhat phase separation, process of hydrolysis and acidification of the organic matter took place in propercondition. It can be concluded that UASB reactor, inoculated by digested slurry requires 84 days for start-up andmay be a robust design for treating low-strength wastewater. Further investigation should be carried out for alonger duration by varying OLR and HRTs to study the steady state and shock load performance. Effect ofvariation in influent parameters on sludge activity and characteristics should be further studied.V. AcknowledgementOne of the authors (K.R.Venkatesh) is thankful to the authorities of the Annamali University forproviding necessary facilities to carry out his research work.REFERENCES[1] Venkatesh, K.R. Biodegradation of lignin from pulp and paper industry effluents, M.E. Thesis. Department of Civil Engineering,Annamalai University, Annamalai Nagar, Tamilnadu, India.2003.[2] Lettinga, G., Field, J., van Lier, J., Zeeman, G. and Hulshoff Pol, L.W. Advanced anaerobic wastewater treatment in the near future.Water Sci. Technol, 1997, 35(10), 5-12.[3] Lettinga, G. Sustainable integrated biological wastewater treatment. Water Sci. Technol, 1996, 33(3), 85- 10 20 30 40 50 60 70 80 90Time, daysBiogasproductioninL/d
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